An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems.
Capacitive-sensing MEMS accelerometer designs are highly desirable for operation in high gravity environments and in miniaturized devices, and due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a two layer capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure two distinct capacitances to determine differential or relative capacitance.
FIG. 1 shows an exploded side view of a prior art three-layer capacitive accelerometer 20 constructed as a conventional hinged or “teeter-totter” type sensor. Capacitive accelerometer 20 includes a pair of static substrates 22 and 24, respectively, having opposed parallel planar faces. Substrates 22 and 24 are spaced from one another and each has a number of metal electrode elements 26 and 28 of a predetermined configuration deposited on one surface to form respective capacitor electrodes or “plates.” In an exemplary scenario, electrode elements 26 operate as an excitation or sensing electrode to receive stimulating signals. The other electrode elements 28 operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements 26 (or 28) operates as both sensing and feedback electrodes when the feedback signal is superimposed on the sensing signal.
A movable element 30, commonly referred to as a “proof mass,” is flexibly suspended between substrates 22 and 24 by one or more rotational flexures 32 situated at elevated attachment points 34 for rotation about a rotational axis 36 to form different sets of capacitors with electrodes 26 and 28. Movable element 30 moves in response to acceleration, thus changing its position relative to the static sensing electrodes 26. This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration. Another set of capacitors for electrostatic rebalance is made up of movable element 30 and feedback electrodes 28. Feedback electrodes 28 function to drive movable element 30 to its reference position balanced between the sensing elements 26 and maintain it there.
When intended for operation as a teeter-totter type accelerometer, a first section 38 of movable element 30 on one side of rotational axis 36 is formed with relatively greater mass than a second section 40 of movable element 30 on the other side of rotational axis 36. The greater mass of first section 38 is typically created by offsetting rotational axis 36 such that an extended portion 42 of first section 38 is formed distal from rotational axis 36. In addition, electrode elements 26 and 28 are sized and spaced symmetrically with respect to the longitudinal axis, L, of movable element 30. Similarly, electrode elements 26 and 28 are further sized and spaced symmetrically with respect to rotational axis 36.
Two- and three-layer capacitive sensors having a teeter-totter configuration suffer from a number of drawbacks. In order to provide more capacitive output and hence better circuit performance (e.g., lower noise) the teeter-totter type capacitive accelerometer calls for a relatively large proof mass. Unfortunately, a large proof mass requires more die area, hence increasing cost and package size. Moreover, a proof mass should rotate as a rigid body. The propensity for a proof mass to deform or bend increases in relation to its increasing size, especially when it is subjected to high acceleration. This deformation or bending causes a non-linearity effect that results in decreased accuracy of the sensor. For example, this nonlinearity can create DC offset in the sensor output and possibly cause dysfunction of the system in which the accelerometer is deployed. A smaller gap between the proof mass and the sensing electrodes or a thicker proof mass may mitigate the problem of deformation and the commensurate non-linearity effect. However, the manufacture of a smaller gap and/or a thicker proof mass leads to manufacturing issues.
A problem particular to the three-layer teeter-totter configuration shown in FIG. 1 is that both the sensing electrodes 26 and the feedback electrodes 28 are clustered proximate rotational axis 36. This configuration is inefficient in that the surface area of extended portion 42, generally termed a shield area, of movable element 30 is unused. Moreover, the surface areas of electrodes 26 and 28 are relatively small due to their clustered configuration about rotational axis 36. A smaller surface area of sensing electrodes 26 results in a lower capacitive output. A smaller surface area of feedback electrodes 28 provides insufficient actuation given voltage levels available from the feedback circuit (not shown).